I have been searching for a generator or 'dynamo' (preferably one built into the hub) to run lights for night riding, but it seems that most are rated for only about 3W (presumably to meet German bicycle standards). I currently have a DiNotte 400R tail light that serves me well, running from a battery. I'd like to add a high-lumen headlight (at least 800 Lumens) and run the whole thing from a generator.

I've measured the average power consumption of the 400R on one of the blink modes to be about 2W. A reasonable ballpark estimate for LEDs is about 100 Lumens per Watt, so I'd need a generator that can source an average power of at least 10 Watts for the head and tail light combined. I have some experience with electronics so an unregulated output in the range of, say, 5-30 Volts is acceptable. Storing energy to supply the power surge when the tail light is 'active' is something I'm confident I can handle. A typical cruising speed would be something like 20 mph (30 km/h).

So: Generator (preferably in the hub) that can output 10 W at a reasonable voltage at 20 mph. Does anyone know of such a thing?

Note that such a generator will consume at least 20...30 watts, which is almost a fifth of your pedaling power at 20 mph - you'll certainly notice the drop in resulting speed.
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ImreSep 27 '12 at 15:09

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The relationship between power and brightness is not liner with LEDs (or any other lighting tech with which I'm familiar). The more energy they consume the more gets wasted as heat. A 4 watt LED is not twice as bright as a 2 watt LED.
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GordonMSep 27 '12 at 15:32

The relationship between power and brightness is not liner with LEDs. The more energy they consume the more gets wasted as heat. This is a little bit confusing. Firstly, power versus brightness is not linear, period. It is logarithmic. Efficiency is a secondary factor: you don't get double the watts of light from a LED when you put in double the watts of electrical energy. (But you can use multiple LEDs in parallel instead to make it work out that way).
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KazSep 27 '12 at 20:48

2 Answers
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Typically you need to purchase a light designed to work with a bicycle dynamo due to the power characteristics of a dynamo (lights need DC current). A well sorted hub dynamo (I personally have had great luck with Schmidt hubs) can produce up to about 6W of power depending on your speed and the design of the hub, which is a reasonable amount of power to work with. The 3W rating you read may have been a minimum power spec at a specified slow speed.

In terms of your overall goal of a 800 lumen front plus rear the answer is yes (with caveats).

Headlights

There are two headlight options (that I know of) that can output 800 lumen:

I researched the Supernova E3 Triple a while back and they seem to do some impressive trickery to get that output. Basically them seem to selectively turn LEDs on and off depending on your speed. The full output will likely be at a reasonably fast velocity (not sure of the exact numbers). Make no mistake either is a pricey option.

Tail lights

If you want to be fully dynamo powered you will need a dynamo specific tail light. Both companies also taillight options and often you can use different brands of dynamo tail lights with a dynamo head light. That said, in this case I would carefully research these possibilities as these high output lights as they are doing some funky things to get that max output, which may limit their compatibility with tail lights.

Note for Commuters

Finally, I would warn against using these high output dynamo head lights for commuting (your ultimate purpose was not stated) as both of these head lights shine above the horizon, like a car high-beam and the 800 lumen output also approaches that a car's headlight. When you shine above the horizon it is great for seeing overhead branches (on the trail), but you blind oncoming vehicles and cyclists. Ever had a car drive at you with their high-beams on?

There are other dynamo LED options which have mirrors designed not to shine above the horizon (similar to a car's regular headlights). These are often quoted as passing German road regulations.

In North America I don't believe there are any regulations for bicycle lights. As such, I have noticed more and more riders using very high output trail lights (e.g. excess of 1000 lumen) on the road and blinding everyone with glee. I have nearly crashed on a bike path as a result of being blinded (I couldn't see anything for a minute afterwards). While not illegal, it is totally inconsiderate.

I may start throwing my metal water bottle at offenders this winter :-)

+1 On the recommendation to avoid using these lights at full power on the road.
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BenzoSep 28 '12 at 19:28

This is good information, but we're going to have to agree to disagree on the issue of power. The ultimate purpose is for night commuting, which passes through some unlit areas with potholes. Car headlamps output 1500Lm to 2000Lm (two incandescent or Xenon on low), so I fundamentally can't get behind the position that a bicycle light should be restricted to below half that (I can back the idea that each element of the light should be below 800 Lm, though)
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nick gSep 29 '12 at 2:46

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@nickg - You didn't read my full argument. It is not that the lights output 800 lumen, it is that all dynamo lights on the market with an output 800 lumen have mirrors designed to shine above the horizon (for trail riding). I had wanted an 800 lumen light as well, but ended up using buying a 350 lumen light because it had the correct mirror. I also have trail lights (750 lumen) and there wasn't much difference in commuting application, because the 350 lumen focuses all the light on the road.
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Rider_XSep 29 '12 at 5:14

Blinding others is both frustrating (for the victim) and dangerous (for the victim)! Use lights that light the road, not the faces of the people on the road.
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VoracSep 29 '12 at 6:29

You may want to considered a hybrid system that utilizes a battery store that the generator feeds into so you can work within the limitations of the hub generator, but supplement it with additional battery light(s).

I'm certain there are electrical experts that could answer if the hub is just a trickle charger for a battery during daylight hours or what the limitations of such a system are based on voltage/amperage/ohms requirements for the various devices. There's also probably some concern around how the battery lights connect to the system.

At a quick look it seems to be a normal hub "dynamo" (not really a dynamo as it's AC) max 3W (so not high power). The clever bit is in the cable, though it's probably only a rectifier and DC-DC converter.
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Chris HOct 2 '14 at 15:01